Polyester flex circuit constructions and fabrication methods for ink-resistant flex circuits used in ink jet printing

ABSTRACT

Flex circuits for use in ink jet printers. In particular, flex circuits for use in ink jet printers that include a polyester material layer supporting a plurality of metal conductors, with the polyester material being a material suitable for use in an ink environment with lower ink permeability and low moisture and ink absorption than polyimide (PI) material. The polyester layer having low ink permeability and moisture and ink absorption to prevent: catastrophic “ink shorting of conductors” failures; adhesion failures; corrosion failures by direct ink contact with the conductors; and material degradation failures that may result if any of the materials are degraded by or react with the ink. Preferably, the polyester material is polyethylene naphthalate (PEN). The polyester base layer is suitable for use in many major flex circuit construction types, including: both adhesive-less and adhesive-based circuits; and one-metal and two-metal layer circuits. Also, a method of producing an improved splice in a continuous Tab Automated Bonding (TAB) style strip of circuits, using a suitable polymer material layer, that is stronger per area than other splices.

CROSS REFERENCE TO RELATED APPLICATION

This application claims the benefit of U.S. Provisional applicationhaving Ser. No. 60/712,363, filed Aug. 29, 2005, entitled “POLYETHYLENENAPHTHALATE (PEN) FLEX CIRCUIT CONSTRUCTIONS AND FABRICATION METHODS FORINK-RESISTANT FLEX CIRCUITS USED IN INK JET PRINTING,” which applicationis incorporated herein by reference in its entirety.

TECHNICAL FIELD

The present invention relates to ink cartridges for ink jet printing,and more particularly to flex circuits including a polyester materiallayer, preferably polyethylene naphthalate (PEN), with low inkpermeability and low moisture and ink absorption to prevent:catastrophic “ink shorting of conductors” failures; adhesion failures;corrosion failures by direct ink contact with the conductors; andmaterial degradation failures that may result if any of the materialsare degraded by or react with the ink.

BACKGROUND OF THE INVENTION

The assembled sub-component or device on printers that enables ink jetprinting and includes flex circuits for electronic interconnections isreferred to either as a printhead or an ink cartridge, with the lattername usually associated with both the printhead and the ink reservoir.Circuitry used in printheads or ink cartridges is almost exclusivelybased on polyimide-based flexible circuit tape, defined as polyimide(PI) dielectric film plus adherent conductors (hereinafter referred toas “PI flex circuits”). The PI flex circuits are used primarily to meetthe following main application requirements: bending (flex to installapplication); die attachment (e.g., wire bonding—ball, stitch and wedgebonding, ultrasonic, thermosonic bonding, thermo-compression bonding,laser welding, conductive adhesive bonding, TAB or tape bonding);adhesive attachment to the cartridge (e.g., lamination, elevatedtemperature curing); high dimensional stability with elevatedtemperature processing; and chemical inertness and compatibility withink.

Current commercially available PI flex circuits for print headenvironments rely on PI as an established, acceptable base flexiblesubstrate. Reasons why PI was selected include its flexibility, itsability to be chemically patterned for backside access, the fine pitchgeometry and other design requirements, and its ability to withstand thetemperatures of processing for the print head environment and thetemperatures experienced during print head operation over its life time.During flex circuit manufacturing, the PI substrate experiencestemperatures of between 100 and 160 C. for anywhere between relativelyshort periods (seconds to minutes) to longer periods (hours) (see PIcolumn of comparative information in Table 1 below in the DetailedDescription section; note: solder reflow is not usually required, but isnoted as an example of a short duration, high temperature process).During ink jet printing operation, the integrated circuit (IC) willreach intermittent localized temperatures required to vaporize the inkof around or above 100 C. So, the flex circuits experience relativelyhigher temperatures during manufacturing than during ink jet printeroperation. PI is considered acceptable for use in the print headenvironment as it has a glass transition temperature (Tg) of 350 to 380C. and an operating temperature of 200 C. (see comparative informationin Table 2 below in Detailed Description section). Polymers having a Tgaround 100 C. or below, however, are considered unacceptable.

Based upon the acceptability of PI as a flex circuit substrate for usewithin a print head environment, other of the factors noted above havebecome important aspects of further developments of flex circuitssuitable for a print head. Many different manners of attaching ICs,printheads, adhesives, coatings, metal conductors, etc. to the substratehave been developed for performance aspects and manufacturability.Recent developments have addressed the effects of the liquid ink, inparticular, on the conductive metal traces, in an attempt to obtaindesired performance of the conductors. The conductors are also becomingthinner and narrower for spacing aspects.

U.S. Pat. No. 5,442,386 (hereinafter “the '386 patent”) describes aprint head or cartridge assembly construction for preventing inkshorting of metal conductors. The patent provides a structure and methodto try to avoid ink interactions with the conductive metal parts of thecircuitry. It is based on PI flex circuits primarily including metalconductors and a PI layer comprising Kapton™ or Upilex™ film, which arepolyimide materials for providing a layer to protect against direct inkinteraction from one side of the metal conductors.

FIGS. 5 and 6 of the '386 patent illustrate different types ofattachments and interfaces of adhesives, coatings, etc. with the flexcircuit that are made during elevated temperature manufacturingprocessing to avoid ink egress, which appear in the prior art (the '386patent, in particular). The top portion of FIG. 5 is a flex circuitsubassembly (with adhesive containing layer 67 already shown as attachedto the flex circuit), which is further assembled by attachment ofanother adhesive 90, contained in the bottom portion of FIG. 5, to theprint head structure shown in FIG. 6.

The '386 patent recognizes the potential effects of ink exposure to themetal conductors and discloses the reliance on the PI layer forprotecting the conductors from one side. In particular, the PI layer 58protects the conductors from the direction that liquid ink exposure isgreatest based upon ink cartridge operation (from above the PI layer 58as illustrated in FIGS. 5 and 6).

The '386 patent's flex circuit (comprising minimally layers 58 and 72)is considered to be an adhesive-less (or 2-layer, e.g.,direct-metallized, sputtering without adhesive to hold the copper tracesto the) PI flex circuit. Adhesive-less flex is a commonly usedcommercial type of flex tape that is supplied by 3M Company of St. Paul,Minn., and 3M is the only company listed as an example of a flex circuitsupplier in the '386 patent. This is in contrast to other“adhesive-based” or 3-layer PI flex circuits, in which the metal circuitlayer is attached to the PI dielectric with an adhesive layer inbetween.

In the '386 patent, the metal traces formed on the PI are protectedalmost wholly by the insulating PI film layer of the flex circuit on theone side facing the ink environment and by an “insulator film” (acoverlayer 67) on the other side of the flex circuit onto which theprinthead cartridge is mounted. The preferred embodiment of the“insulator film” is described as a complicated, 3-layer structure (layer67 equals layers 158, 154 and 156 in FIGS. 13 a-d) that is laminated tothe flex circuit to cover most of the conductors 72. The 3-layerstructure is described as comprising: an adhesive (that attaches to theconductor and substrate portions of the flex circuitry); a polyethyleneterephthalate (PET) polyester layer (as a middle layer); and anotheradhesive (that attaches to the main body of the print cartridge).

Features, such as an opening, are patterned in the PI layer 58 (theexample process stated in the '386 patent is laser ablation) and throughthe 3-layer insulator film 67 (a punching patterning method isdescribed) to allow for spanning certain conductor traces so that theyare partially exposed for IC bonding. After IC bonding, encapsulation ofthe remaining, uncovered areas of metal conductors with other insulatingcoatings (e.g., “adhesive beads,” “adhesives,” “encapsulant beads”) isused to avoid any direct contact of the ink to the circuitry, which mayflow in the vicinity of the conductors.

The important “insulator film” (or coverlayer) properties are summarizedin the '386 patent as: material handling ease; adhesion to PI tape;adhesion to print head cartridge; and, fluidic sealing of conductorsfrom the ink. The PET layer has the following properties, whichcontribute to why the 3-layer insulator film is the preferred embodimentin the '386 patent: it has better structural integrity as compared withthe two adhesives in the 3-layer structure, resulting in handling ease(e.g., ease in the punching patterning method, keeps structuralintegrity while adhesives can be softened at higher temperatures duringbonding operations); it has or develops no large holes or voids duringprocessing, such as other materials like hot melts might develop, whichwould allow for ink flow through the voids to reach the conductors; and,it has ability to withstand moderate temperatures (another advantageover “hot melts”).

Besides having a 3-layer insulating film assembled separately to flexcircuits such as in the '386 patent, flex circuits with an adherentcover coat or coverlayer over the conductors (e.g., the coverlayer takesthe functional place of layers 158, adhesive, and 154, structural,hole-free layer in FIGS. 13 b-d) can be supplied for assembly. Aseparate adhesive (like layer 156) can be used to attach it to the otherside of the assembly. One example of suitable cover coat material is thesubject of Japanese published patent application no. HEI 10[1998]-158582describing a “Protective Coating and Use of Liquid Thereof for InkCartridges.”

In addition to adhesive-less PI based circuits, TAB-type circuits (ortape-automated bonding “tapes”) based on adhesive-based PI areappropriate for some loosely toleranced printer flex circuit designs. Ina typical TAB process the adhesive and PI are patterned together by useof a metal die or other method, then after lamination of metal (usuallycopper) to the adhesive side, the metal is patterned (usually bychemical etching).

The use of a wide variety of possible ink materials have been developedfor ink jet use including the use of solvents such as ionic compounds(e.g., high, neutral and low pH, see patents: Japanese Kokoku Patent No.3097771, U.S. Pat. Nos. 4,853,037, 4,791,165 and 4,786,327, EuropeanPatent No. 259001, and U.S. Pat. Nos. 4,694,302, 5,286,286, 5,169,438,5,223,026, 5,429,860, 5.439,517, 5,421,871, 5,370,730, 5,165,968,5,000,786 and 4,990,186). Solvents within such inks place severeconstraints on the choice of materials for both the flex circuit basematerial and any insulator film because it is important that the basematerials will not be dissolved by the ink. It is desirable to preventink from interacting with the conductor traces in order to attain longprint head life. The above-noted list of reference patents disclosingink materials is set out in Japanese published patent application(Patent Journal (A) Kokai Patent Application no. HEI 10[1998]-158582).

In the '386 patent, ink “shorting” mechanisms are not specificallydescribed, but the ionic or polar nature of inks, if present between anytwo adjacent conductors with different voltage potentials, might renderit a conductive medium, causing some undesirable level of current toflow between the conductors resulting in electromigration, also calledcathodic-anodic filament growth, CAP or dendritic growth. It is wellunderstood that the CAF reliability issue becomes important for any flexor hardboard circuit, IC package or assembly where there is moisturedegradation. Moreover, in the presence of moisture, it is known that thedriving force increases with the voltage difference between conductorsand with the concentration of ionic species in the region betweenadjacent conductors. As with the presence of moisture, similarly must bethe case with liquid inks. Indeed, the '386 patent provides that futurehigh voltage levels, faster speeds and/or de-multiplexing circuitrydesigns might lead to use of high current supply voltages and lowcurrent control signals to be carried by the conductors and thus resultin severe rather than moderate effects of shorting on the operation ofthe print head.

The aggressive chemical nature of the ink might cause the followingtypes of catastrophic “ink shorting of conductors” failures: adhesionfailures (metal circuitry trace-to-PI, insulator-to-PI andtrace-to-insulator); corrosion failures by direct ink contact with theconductors if not covered by flex circuit base material or othermaterials; and material degradation failures that may result if any ofthe materials are degraded by or react with the ink (e.g., dissolution).These potential failures could occur at any time during operation of anink jet print head.

SUMMARY OF THE INVENTION

Shortcomings of the prior art are overcome by the present invention inthat a suitable substrate material for a flex circuit usable in a printhead environment should be selected not only for temperature constraintsbut also to guard against the failure mechanisms (corrosion,electromigration, CAF, ink reaction and adhesion loss). In particular,low moisture and ink absorption and permeability are importantproperties of flex circuit base materials, coverlayers and cover coatsfor flex circuits and flex circuit assemblies used in ink cartridgeapplications. Each of these properties can be selected to lower theconcentration of inks in the vicinity of the conductors.

As a flex circuit insulator and base material, PI is consideredacceptable for print head use because of its higher heat tolerance.However, PI has limitations because of poor ink compatibility, which arebelieved to arise because of its higher absorption of water (thuspresumably also ions present in ink aqueous, polar solutions). Certainpolyesters, e.g., PET, are limited by their thermal properties (seecomparative information in Table 1 below in Detailed Descriptionsection) although superior as compared with PI with respect toabsorption and permeability properties. In accordance with the presentinvention, polyethylene naphthalate (PEN), a particular polyester, canadvantageously be used as a base insulator for flex circuits becausePEN, offers considerably better ink soak trace peel adhesion, lowmoisture absorption and other improved ink resistant properties and haslower cost than PI. Also, PEN uniquely has excellent dimensionalstability and high temperature stability among currently developedpolyesters required for ink cartridge assembly. PEN-based flex circuitsfabricated with different methods meet the current criteria for printhead use.

The present invention preferably utilizes a PEN material base layer, incontrast with an “insulating” or “sealing” material covering a PI basecircuit material, because of the discovered importance of low moistureand ink permeability and absorption. Although the presence of anybarrier material is helpful to avoid direct ink contact, thepermeability and absorption properties of the materials are moreimportant material properties of the base layer for performance overtime. According to the present invention, PEN, or other polyesters thatare known or may be developed with similar material properties, areutilized instead of PI as a flex circuit base material in ink useenvironments because they impede the transport of ink through it betterthan most PIs and they have a lower moisture absorption. Although, PENand other polyesters are also suitable for use as coverlayers.Coverlayers have different functions than base materials for flexcircuits. It is more important for the base material to have inkresistant and low ink permeability and absorption properties than thecoverlayer or cover coat, because the base material faces and directlycontacts the ink (see layer 58 in FIG. 6). A coverlayer can, however,benefit from having similar properties. A coverlayer does not contactthe ink directly as it can be provided in contact with other adhesivesand coatings and is located further in the interior of the print headassembly.

The present invention utilizes a polyester base layer (preferably PEN)suitable for use in an ink environment with lower ink permeability andlower moisture absorption than PI and is suitable for use in many majorflex circuit construction types, including: both adhesive-less andadhesive-based circuits (includes TAB-type circuits); and one-metallayer and two-metal layer circuits. The preferable use of PEN alsopermits the use of and method of producing an improved splice that isbased on a welding of the PEN material that cannot be achieved with thePI-based prior art and is stronger per area than current splices andsplice methods.

One aspect of the present invention is a flex circuit for use in an inkjet printer, the flex circuit comprising a flexible substrate comprisinga polyester material layer supporting a plurality of metal conductorsadhered along at least a portion of a first side of the substrate, thepolyester material comprising a material suitable for use in an inkenvironment with lower ink permeability and moisture absorption than PImaterial. Preferably, the polyester material of the substrate comprisesPEN. Another embodiment further comprises at least one opening providedthrough a suitable polyester layer for providing access to at least oneconductor. Yet another embodiment further comprises a metal access padadhered on the first side of a suitable polyester substrate layer withthe plurality of metal conductors, the metal access pad being accessiblefrom a second side of a suitable polyester substrate layer through apatterned opening through the suitable polyester substrate layer, andwherein at least one metal conductor is also accessible from the secondside of the suitable polyester substrate layer by way of another openingthrough the suitable polyester substrate layer. A further embodimentfurther comprises at least one metal conductor adhered along at least aportion of a second side of a suitable polyester substrate layer andthat is electrically connected through a metal via extending through thesuitable polyester substrate layer to at least one of the metalconductors on the first side of the suitable polyester substrate layer.Another embodiment is a flex circuit further comprising an adhesivelayer between the suitable polyester substrate layer and at least one ofthe metal conductors for adhering them together, wherein at least one ofthe metal conductors may be adhered to the suitable polyester substratelayer without an adhesive layer in between.

A second aspect of the present invention is a method of making a flexcircuit for use in an ink jet printer, the method comprising the stepsof providing a flexible substrate including a polyester material layerand adhering a plurality of metal conductors to one surface of thesubstrate, wherein the polyester material is suitable for use in an inkenvironment with lower ink permeability and moisture absorption than PImaterial. Preferably, the polyester material of the substrate comprisesPEN. Another embodiment further comprises the step of patterning atleast one opening through the suitable polyester layer for providingaccess to at least one conductor. Yet another embodiment furthercomprises the step of adhering a metal access pad on the first side ofthe suitable polyester substrate layer along with the plurality of metalconductors, the metal access pad being accessible from a second side ofthe suitable polyester substrate layer through a first opening patternedthrough the suitable polyester substrate layer, and patterning a secondopening through the suitable polyester substrate layer so that at leastone metal conductor is also accessible from the second side of thesuitable polyester substrate layer by way of the second opening. Afurther embodiment further comprises the step of adhering at least onemetal conductor along at least a portion of a second side of thesuitable polyester substrate layer and electrically connecting the metalconductor on the second side by way of a metal via extending through theopening of the suitable polyester substrate layer to at least one of themetal conductors on the first side of the suitable polyester substratelayer. Yet another embodiment further comprises the steps of providing alaminate of the suitable polyester substrate layer and an adhesivelayer, patterning the laminate to provide at least one access openingthrough the laminate, adhering a metal layer to the suitable polyestersubstrate layer by way of the adhesive layer, and then patterning themetal layer to create the plurality of metal conductors. A furtherembodiment further comprises the step of providing an adhesive layerbetween the suitable polyester substrate layer and at least one of themetal conductors for adhering them together, wherein at least one of themetal conductors may be adhered to the suitable polyester substratelayer without an adhesive layer in between.

A third aspect of the present invention is a print head for use in anink jet printer comprising a printer and an ink cartridge and a flexcircuit connected electrically to the IC, the flex circuit comprising aflexible substrate comprising a polyester material layer supporting aplurality of metal conductors adhered along at least a portion of thesubstrate, the polyester material comprising a material suitable for usein an ink environment with lower ink permeability and moistureabsorption than PI material.

A fourth aspect of the present invention is a method of joining aplurality of flex circuits together in series comprising the steps of:providing a plurality of unconnected flex circuits, each having aflexible substrate including a thermoplastic polymer material layer,wherein the thermoplastic polymer material is suitable for use in an inkenvironment with lower ink permeability and moisture absorption than PImaterial, and each flex circuit further having a plurality of metalconductors adhered to one surface of the substrate; and splicing oneflex circuit to a second flex circuit by overlapping at least a portionof the first and second flex circuits together and applying heat andpressure sufficient to thermally bond the first and second flex circuitstogether in series. Preferably, the thermoplastic polymer material ofthe substrate comprises a polyester. More preferably, the thermoplasticpolymer material of the substrate comprises PEN. In another embodiment,the first flex circuit is combined with one or more additional flexcircuits having the thermoplastic polymer material substrate layer incommon.

A fifth aspect of the present invention is a method of joining aplurality of flex circuits together in series comprising the steps of:providing a plurality of unconnected flex circuits, each having aflexible substrate including a polymer material layer, wherein thepolymer material is suitable for use in an ink environment, and eachflex circuit further having a plurality of metal conductors adhered toone surface of the substrate; splicing one flex circuit to a second flexcircuit by overlapping at least a portion of the first and second flexcircuits together and applying heat and pressure sufficient to thermallybond the first and second flex circuits together in series; andinserting a strip comprising an adhesive on the overlapped portionbetween the first flex circuit and the second flex circuit prior tothermally bonding the first and second flex circuits together.

BRIEF DESCRIPTION OF THE INVENTION

FIG. 1 shows a perspective view of a 1 ML adhesive-based PENconstruction;

FIG. 2 shows a perspective view of a 2 ML adhesive-based PENconstruction.

FIG. 3 shows a perspective view of a “near-invisible” splice of a tapeof PEN circuits;

FIG. 4 shows the same perspective view of the “near-invisible” splice ofFIG. 4 with the addition of a narrow film strip;

FIG. 5 shows a bar graph of percent peel strength retained in high pH(>8) ink at 60 C. at weeks 0-8 for samples of PEN and PI applied to¼-inch wide circuit traces; and

FIG. 6 shows a bar graph of percent peel strength retained inneutral/low pH (<7) ink at 60 C. at weeks 0-8 for samples of PEN and PIapplied to ¼-inch wide circuit traces.

DETAILED DESCRIPTION OF THE INVENTION

The invention is directed to articles and methods, and involves anadhesive-based or an adhesive-less flex circuit construction, includingconductors adherent to a polyester base layer, polyethylene naphthalate,hereinafter PEN. The invention is based on PEN polymer, or other knownor developed polyesters with similar permeability and absorptionproperties, as discussed below, and preferably having low heat shrinkagesuitable for dimensionally-stable flex circuitry. The followingdiscussion is primarily directed to the use of PEN as a suitablepolyester having desired properties in accordance with the presentinvention, but it is contemplated that other polyesters may be knownand/or developed that would also be suitable. For example, polyethyleneterephthalate (PET) shares similar properties suitable for an inkenvironment (i.e., moisture and ink permeability and absorptionproperties) but is not as thermally stable. However, for certain lowertemperature applications, PET could function effectively. Moreover,other PET variations, or other polyesters, may be known or developedhaving desired ink environment properties with higher or improvedthermal stability that could be used in similar low or highertemperature applications. PEN is much less expensive than PI (bothadhesive-less PEN is less expensive than adhesive-less PI, andadhesive-based PEN is less expensive than adhesive-based PI). The flexcircuit constructions, in accordance with the present invention, may becoated with an insulator or cover material that is strongly adherent tothe PEN/adhesive or PEN, respectively. The insulator could be a dry film(e.g., cover coat, ink or coverlayer, through non-vacuum or vacuum-basedlamination), a liquid screen printable, or slot-die or curtain coatedinsulator material. A PEN-based flex circuit can be assembled and fit inwith other parts of an ink cartridge, such as that described in, forexample, the prior art construction of the '386 patent (where aninferior PI-based flex circuit is used).

“Adhesive-based” or 3-layer flex circuits, as in FIG. 1, mean that themetal circuit layer is attached to the PEN with an adhesive layer inbetween. As shown in FIG. 1, adhesive-based flex circuits have apatterned metal circuit layer 10, an adhesive layer 20 adjacent to themetal circuit layer 10, and a layer of PEN 30 adjacent to the adhesivelayer 20 and opposite the metal circuit layer 10. “Adhesive-less” or2-layer flex circuits mean that the metal circuitry directly contactsand is adherent to the PEN without any adhesive, which is similar to theconstruction of adhesive-less PI flex circuits described in the '386patent, except that PEN material is used as the base layer for superiorperformance for ink jet print heads. The circuit design acceptable forinkjet print heads preferably includes both frontside and backsideconductor access such as facilitated by patterning the PEN and anyadhesive to achieve extended conductor traces over removed or vacantdielectric regions. Examples of flex circuits in accordance with thepresent invention are shown in FIGS. 1 and 2. FIG. 1 shows a one-metallayer (“1 ML”) construction having one metal circuit layer 10 andbackside access terminals 40. FIG. 2 shows a 2-metal layer (“2 ML”)construction with one metal layer circuit layer 10 on the upper surfaceof the flex circuit and a second patterned metal circuit layer 70(indicated by the dotted lines 70 on the lower surface of the layer 50)on the lower surface of the flex circuit. The two metal circuit layersare connected with conductive metal vias (indicated by dotted lines 60that extend through the layers 20, 30, 50) that connect the first metalcircuit layer 10 to the second metal circuit layer 70, A corresponding 1ML adhesive-less PEN construction would not include the adhesive layerin the middle, as the conductors can be directly adherent to PEN. The 2ML adhesive-less PEN construction would not include the two adhesivelayers (20 and 50 in FIG. 2) on either side of the PEN layer 30 in themiddle, as the conductors can be directly adherent to PEN. In each case,the metal surfaces in the constructions can be either fully or partiallygold plated or finished with other noble and bondable metals, which caninclude the patterned, unsupported traces meant for later IC attachment,backside access terminals for electrical contact and back, front andsides of the patterned metal features.

In the adhesive-based PEN embodiments (metal/adhesive/PEN), a metallayer of an unpatterned metal/adhesive/PEN laminate raw material roll orsheet can be chemically etched to fabricate multiple conductor traces byusing a photomask and a set of process steps (photoresist-basedmaterial: application by lamination of a film or liquid coating, expose,develop and later remove after etching). A laminate raw material ispreferably selected for survival of the materials and interfaces for theharsh ink environment of ink jet printing and for the high temperatureof flex circuit fabrication and assembly manufacturing process stepsthemselves, as described previously in the Background section. Thus,metal foil and adhesive selection is preferably based on tests such asare described in the Examples including a maximization of metal adhesionin the laminate before and after exposure to ink. For the metal layer ofthe laminate raw material, copper foils with all three of criticaladhesion, barrier, and stabilization treatments are preferred forPEN-based flex circuits in the ink jet printing application, based onsuccessful testing of different foils with all these three treatments(see Examples 2, 3 and 5). Adhesion treatments increase the strength ofthe adhesive-copper bond and can comprise: (a) nodule ormicro-roughening treatments that add surface area; and (b) adhesionpromoter treatments like a silane coupling agent that improves chemicalbonding. Barrier treatments give increased reliability in moist or hightemperature environments similar to the ink printing environments (inkconstituents are commonly polar in nature like water and many are waterbased) and typically comprise a known type of brass or zinc treatment(e.g., up to 120 nm thickness). Stabilization treatments inhibitcorrosion and typically comprise the use of an oxide, chromium orchromium alloy (where Cr is in +3 valence state, typically less than 10nm thickness). For the adhesive, high ink and moisture (being a polarcompound like ink constituents) resistance have been found to bepreferred properties. Moreover, based on an ordering based on inkresistance tests (see Example 3 for the description of a 60 C. soakingof parts in ink for 1000 hours) for some cover coat materials applied toPI adhesive-less flex circuits, an adhesive material designation Type L(epoxy, see IPC Spec 4204, May 2002) is expected to outperform both M(acrylic) and P (butyral phenolic) as used in adhesive-based PENlaminates. The discussion in the Example 2 section describes tests wherethe retention of peel strength of metal to PEN and PI base materials wasmeasured, but in the cover coat ink resistance tests the PI flexcircuits cover coated with different cover coat chemistries exhibited aranking with respect to delamination of different cover coats; acrylic-and butyral phenolic-based cover coats delaminated much more quicklythan epoxy-based ones, some of which survived after 1000 hours. Resultsdetailed in Examples 2, 3 and 5 indicate PEN laminates with variousadhesive chemistries, including modified epoxy and polyester-epoxyblends, performed successfully in different tests that evaluated inkresistance.

Moreover, based on PEN circuit fabrication with commercially availablefoils and laminates (JTC Flex™ silane-treated, micro roughened, foilwith zinc—chromium layers, commercially available from Gould ElectronicsInc., located in Chandler, Ariz., U.S.A.; PEN laminates G1910 and G1965that use polyester-epoxy blends for adhesives (commercially availablefrom Multek Flexible Circuits Inc., Sheldahl Technical MaterialsDivision, located in Northfield, Minn., U.S.A.), and DuPont-Teijin Q83™,for the PEN base material, foil (commercially available from DuPontTeijin Films U.S. Limited Partnership, located in Hopewell, Va., U.S.A.)with PEN laminate GTS 5670 (commercially available from GTS FlexibleMaterials Ltd., located in the Berkshire, United Kingdom) that use amodified epoxy adhesive) with high metal peel strengths and peelstrength retention after exposure to humidity and temperature, it isfurther contemplated that (a) other foils with similar micro rougheningtreatments, silane-coupling or other adhesion promoting treatments,zinc—chrome barrier and stabilization treatments and (b) other Type Land N adhesives, are preferable. However, the present invention is notlimited to those specified foils and laminates as other types of foilswith none or one or more of the above-noted treatments and adhesiveswith other designations can be suitable for use in PEN-based flexcircuit ink jet printing applications. Also, other IPC adhesivedesignations (see IPC Spec 4204, May 2002), including Types R and Y, mayalso be acceptable without limitation, although such adhesive laminationtemperature with copper foil may be limited to the softening temperatureof the PEN, or similar polyester, based material. Circuits made from onesource of PEN (PEN films commercially available from DuPont Teijin FilmsU.S. Limited Partnership located in Hopewell, Va., U.S.A.) that wereannealed after being formed as a film to improve dimensional stabilityhave been described previously (such as those commercially availablefrom Multek Flexible Circuits Inc., Sheldahl Technical MaterialsDivision, located in Northfield, Minn., U.S.A.). Another example of aPEN film that is commercially available is the Skynex® NX10L film,commercially available from SKC Co., Ltd. Both have been found to haveabout the same low ink permeability and moisture absorption compared toPI (see Example 1). However, the present invention is not intended to belimited to just those PEN film sources and PEN laminate manufacturersthat were tested. For example, it is contemplated that the PEN raw filmcould be made and laminated by other methods and can be formed byvarious means (e.g., extrusion, blow molding, tubular film extrusion,etc.), providing that the films achieve sufficient dimensional stabilityto hold tolerances for flex circuits (preferably better than +/−0.3%,IPC).

To fabricate backside access features, a PEN material layer and adhesivemay be patterned successfully with methods such as (or an appropriatecombination of) laser ablation, chemical etching, plasma etching (e.g.,use of oxygen or oxygen-CF4 gas mixtures), chemical and/orelectrochemical cleaning and mechanical cutting or stamping operations(e.g., making use of metal dies; see also Example 4). Laser ablation ofboth PEN and adhesive layers sequentially in the same patterninglocations is a preferred method for producing both backside access andvacant dielectric regions for unsupported metal conductors in 1 MLdesigns (see FIG. 1 and Example 4) and small via holes in 2 ML designs(see FIG. 2; in this case sequential layers in the stack-up of metal,adhesive, PEN, adhesive and metal in the raw material laminate could belaser ablated and cleaned). However, adhesive can also be paired eitherwith PEN, with metal or by itself, and the layers can be patternedtogether or separately with an adhesive lamination step inserted at anappropriate time in the process. For example, for 1 ML designs, thePEN-adhesive can be patterned first and then attached (e.g., bylamination) to the metal. Then the metal can be patterned such as byvariation on the TAB process described earlier, but where PEN replacesPI. In order to remove any adhesive by-products that may have beenincompletely removed by the laser and/or plasma, chemical cleaning andmicroetching (e.g., with a sulfiric-based or other acid-based solution)techniques, as themselves are well known, are preferably conducted priorto any surface finishing (e.g., gold plating).

For an adhesive-less PEN construction, direct metallization can beaccomplished by vacuum deposition techniques or high temperature copperfoil laminations near the melting point of the PEN. Then, the metal canbe patterned by additive, semi-additive or subtractive processes using aphotoresist. Sputtering of metal is specifically contemplated as aneffective manner to metalize PEN or other suitable polyester material assuch procedure is known to be effective in metallizing PI in makingadhesive-less PI in production. However, evaporation and other vacuumtechniques are also believed to be possible and are expected to beusable. Example 4 below further demonstrates examples of certain foillaminations that are useable in accordance with the present inventionand that suggest the ability to create similar foil laminations.

It is also contemplated that a unique, low-cost, semi-additive-based orsubtractive-based process flow could progress from a raw materialproduced by laminating PEN directly or indirectly with adhesive (e.g.,DuPont Q83™ film, commercially available from DuPont Teijin Films U.S.Limited Partnership, located in Hopewell, Va., U.S.A.) and with a thincopper foil, which has a separable interface between a thin, few-micron(e.g., 1 to 5 micron) copper layer and a thicker sacrificial copperlayer that can be separated after lamination. As one specific example,it is contemplated that single-sided (4 micron or 35 micron copper/PEN,where 35 micron copper is a commonly available foil) or double-sidedmaterial (1-4 micron copper/PEN/1-4 micron copper or 35 microncopper/PEN/1-4 micron copper) could uniquely be produced for the 1 MLand 2 ML constructions, respectively (see FIGS. 1 and 2). In contrast,commercial raw material with PI (e.g., Kapton™ H film commerciallyavailable from E.I. du Pont de Nemours and Company, located inWilmington, Del., U.S.A) is produced from vacuum-metallization or bycasting PI on metal foil. Analogous to the cast PI process, it iscontemplated that PEN or other raw material can be produced bydepositing from solution or other means (e.g., casting) PEN polyester onmetal foil as a viable source of polyester (PEN)/metal substrates forcircuitizing into 1 ML flex circuits useful for ink jet printing.

For either vacuum-based or lamination-based processes, a tie coat or tielayer may be removed by chemical means. For sputtering, a chrome tiecoat can be utilized (as in the Examples below), but other sputtered tiecoats like NiCr, monel and others are contemplated, which may havebetter corrosion resistance to inks.

For lamination-based processes for making adhesive-less based PENcircuits, copper foils with adhesion, barrier and stabilizationtreatments (as discussed above with regard to adhesive-based PEN) aredesirable for moisture-resistance and ink resistance, as discussed abovefor similar reasons as described for adhesive-based PEN applications.Thus, preferred foils with micro roughening treatments, silane-couplingor other adhesion promoting treatments, zinc—chrome barrier (forreliability in moist environments) and stabilization (or antioxidation)treatments (like those provided by JTC Flex™ foil, commerciallyavailable from Gould Electronics Inc.) are preferably combined withdimensionally stable PEN or other polyester films (like DuPont-TeijinQ83™ film, commercially available from DuPont Teijin Films U.S. LimitedPartnership, located in Hopewell, Va., U.S.A.) in order to achieveeffective metal peel strengths and peel strength retention afterexposure to inks.

The same or a combination of some of the same patterning methodsdescribed above in describing adhesive-based PEN can be used for PENalone (e.g., laser ablation, plasma ashing, chemical cleaning andmechanical patterning), but also chemical removal of PEN material hasbeen demonstrated. As with the adhesive-based construction, laserablation is a proven and preferred method for producing small (e.g., 25to 75 micron diameter) via holes to conserve area in 2 ML designs, buteven chemical and mechanical (e.g. punching) removal of larger holes onadhesive-less PEN for vias are contemplated.

Whereas chemical removal of both PEN and adhesive is difficult becausetwo chemistries are likely needed to etch two different materials,chemical etching of PEN at reasonable reaction rates can be accomplishedby controlled cleavage of the PEN polymer chain in unmasked areasexposed to chemical reactants in conjunction with either a backsidemetal (preferred, or possibly a photoresist) mask. A 1 ML adhesive-lessPEN construction can be fabricated from a double-sided metallized rawmaterial with the frontside patterned for circuitry and the backsidepatterned as a sacrificial metal mask. PEN is believed to be unzippedinto soluble, single, naphthalate-ester fragments with most relativelynon-volatile, water-soluble organics with a single-functional OH groupand other functional group(s) to increase the boiling point (called“modified simple alcohols” ), but not—COOH acid groups, because of theinterchange reaction pattern of polyesters is by alcoholysis and not byacid groups (see P. J. Flory, “Chapter 3, Condensation Polymerization,”in Principals of Polymer Chemistry, Cornell University Press, Ithaca,N.Y., pp. 69-105, incorporated herein by reference). The modified simplealcohol (e.g., mono ethanol amine (MEA)) should be selected to have amoderate boiling point so as not to be removed during processing aroundthe boiling point of water where reasonable reaction rates are achieved(thus MEA, a modified alcohol, is preferred above ethanol or apropanol). Larger alcohols (e.g., butanols and pentanols) have higherboiling points, but would not be preferred because of lower solubilityof themselves and the corresponding naphthalate ester product in aqueoussolutions. Multifinctional alcohols (or glycols) may also unzip thepolymer and have the advantage of a greater number of reactant OH groupsand a greater solubility in water, but may be subject to undesirableside polymerization reactions. The solubility of the naphthalate-esterproduct of the cleavage reaction is also important to the choice of themodified simple alcohol.

Chemical etch rate can be increased to useful levels by catalysis ineither basic solutions (e.g., NaOH or KOH) or likely also in acidicsolutions (e.g., sulfuric-based or other), consistent with polyesteracid-base-catalyzed, trans-esterification mechanisms relying on “thepolar nature of the carbon-oxygen double bond and the ability of thecarbonyl oxygen atom to assume a formal negative charge” (see M. P.Stevens, “Chapter 10: Polyesters” in Polymer Chemistry: An Introduction,Addison-Wesley, Reading, Mass., pp. 251-275, also incorporated herein byreference). As it is understood that a high PEN etch rate can beachieved in highly basic solutions with certain concentrations ofMEA/KOH and MEA/NaOH, “presumably increasing the nucleophilicity of thealcohol by formation of the alkoxide anion” (see Stevens' description oftransestification mechanisms of the base catalyzed reaction), it isfurther believed that acid catalysts with MEA or other modified simplealcohols will “coordinate the carbonyl oxygen and thus enhance theelectrophilic character of the carbonyl carbon” (see Stevens'description of transestification mechanisms of the acid catalyzedreaction).

A preferred PEN chemical removal method to produce well defined, angledPEN sidewalls can be based upon a circuit processing technique utilizingdouble-sided metal covering PEN and by using backside metal patterningto define a metal mask that can be used in a PEN patterning removalstep. Initially, frontside circuitry and backside mask patterns can beetched at the same time using a photoresist method. The frontsidecircuitry can be protected with a blanket exposure of photoresist duringa patterned removal of PEN based on the backside mask pattern. Thebackside metal thickness is preferably thin and the metallization methodused is preferably a low cost method so as to minimize both the cost ofthe etching step to remove the sacrificial metal and the overallprocessing costs. Lamination of thin metal foils to PEN can beeffectively accomplished by using thin, separable copper foil (e.g.,Olin Corporation, Brass Division's (located in East Alton, Ill., U.S.A.)CopperBond® XTF™ foil; also, see previous discussion wherein a thin foilportion can remain after separating an interface after lamination) andsuch laminations are expected to be less expensive than second-sidevacuum metallization processes that are currently used on PI (e.g., 25micron copper/tie coat/PI/tie coat/4 micron copper adhesive-less rawmaterial as is commonly used).

A further discussion regarding the fabrication methods described andsuggested above and superior ink resistance of PEN- or otherpolyester-based flex circuits versus PI-based flex circuits is found inthe Examples section below.

Table 1, below, shows a comparison of polyesters including PET and PENto PI as to temperature suitability for making flex circuits usable forink jet printing. While PET as currently available is generally lessthan acceptable, primarily due to temperature exposures ofmanufacturing, PEN is highly acceptable for temperature criteria asillustrated in comparison to PI, while being significantly better thanPI with respect to permeability and absorption (as detailed below). Withthe advent of improved or different manufacturing steps that may includelower temperature processing, PET may also be acceptable for processingas flex circuits with superior permeability and absorption properties ascompared with PI (also detailed below). Likewise, other polyesters maybe known or developed with effective properties for processing andenvironmental usage with superior permeability and absorption propertiesas compared with PI for ink jet applications. TABLE 1 Common ThermalExcursions for Circuits and Acceptability Expected for CircuitDielectric Material Based on Physical Properties of Select Dielectrics.Dielectric Material PET PEN PI Cover Coat Cure Not acceptable AcceptableAcceptable 130 C.-160 C., 10 to 90 minutes over whole range over wholerange over whole range Assembly (adhesive, encapsulants, cover lays,Acceptable in lower Acceptable Acceptable other coatings) 110 C.-160 C.,10 to 60 minutes, range over whole range over whole range IC attach, 150C.-160 C. (typical), but also Acceptable in lower Acceptable overAcceptable above and below, seconds to 10 s of seconds range whole rangeexcept over whole range high extremes Peel After Solder Float (%retension) Fail 8.5 lbs/in 12 lbs/in (IPC TM 650 #2.4.9, method D, 204C. for 5 sec) (100%) (100%)See Table 2 for comparison of PI, PEN and PET properties

The ability of PEN-based circuits, in particular, to pass the shortduration solder float test (see Table 1: test performed for 5 seconds at204 C.) with minimal impact on peel retention (percent of the forceretained after versus before solder exposure), demonstrates PEN'ssuitability similar to that of PI along with PEN's superiority over PETwith respect to thermal processing. Moreover, this ability alsoevidences PEN's adequacy in surviving many short to medium duration(seconds to tens of seconds at least) manufacturing temperatureenvironments during printer flex circuit assembly that are above theflex circuit's continuous use operating temperature (above 160 C.). Theuse of PEN-based flex circuits would, for example, potentially excludeonly the most extreme IC attach/bonding processing conditions (bondingtemperature extremes were reported to reach above 300 C. in G. Harman'sreview of the many potentially usable methods that are mentioned abovein the Background section for IC attachment (Wire Bonding inMicroelectronics: Materials. Processes, Reliability and Yield,McGraw-Hill, N. Y., 2^(nd) ed., 1997). As such, those extremetemperature techniques can easily be avoided by selection among the manyother, non-extreme processes. Indeed for the IC attach step, the basematerial usually does not come into contact with the heat source, exceptindirectly by conduction through unsupported traces, and an energy pulseis usually short (order of microseconds to milliseconds), thus the basematerial itself usually reaches temperatures much lower than the actualbonding temperatures. Thus, higher temperature resistance of PI ascompared to that of PEN is an unnecessary property, and PEN-basedcircuits and other high temperature resistant polyesters (albeit thosewith higher resistance than PET as tested) have sufficient temperatureresistance to withstand current thermal processing steps.

Table 2 below provides a comparison of certain physical properties ofPET, PEN and PI. These properties are relevant and useful with regard tothe Examples below, in particular with regard to Example 1. Also, theTable provides, for example, that the water absorption percentage of PENis much lower than that for PI. Additionally, the water absorptionpercentage for PET is close to that for PEN, indicating that otherpolyesters, such as PET, are also suitable materials for the presentinventive flex circuits. TABLE 2 Physical Property Comparisons of FlexCircuit Dielectric Materials. Dielectric Material Physical Property Unitof Measure PET PEN PI Thickness mils 1 to 5 1 to 5 1 to 5 ThermalShrinkage % 0.2 to 1.5 0.08 to 0.5  0.03 to 0.18 {IPC TM 650, 2.2.4A,150 C., 30 min} Coefficient of Humidity Expansion ppm/% RH 10-11 10-115-16 Water Permeability g/m²/day 21-26  6-10 4-28 {JIS K 7129 method B}Water Absorption % 0.4-0.8 0.3-0.6 1.8-3.7  {ASTM D570-01, 23 C., 24hrs} Oxygen Permeability cc/m²/day 55  21  4-114 {ASTM D1434} MeltingPoint C 260-264 269-272 NA Glass Transition Temperature (Tg) C 69-78120-122 350-380  {IPC TM 650 2.4.24.2, DMA method} Maximum ContinousOperating Temperature C 100-105 160 200 {UL7466} Alkali Resistance,weight loss after 5minutes % nil nil 4-26 in 1N KOH at 40 C.

In accordance with the present invention, a dyed adhesive can be usedwith adhesive-based PEN for creating flex circuits for the purposes ofincreased process throughputs (e.g., laser ablation rate) and aestheticneeds. A dyed adhesive having a color that corresponds to the wavelengthof the laser, results in the laser ablation process more effectivelyremoving material more quickly. In the Examples below, a dye was addedto at least one type of adhesive used, and it did not negatively affectperformance of the adhesive during the ink resistance testing. Frominformation as shown in Examples 2, 3 and 5, the use of dye withinadhesive is believed to be suitable for use in flex circuits for ink jetprinter applications.

Unlike PI as used in prior art adhesive-less based or adhesive-basedflex circuit (having trade names Kapton™ E-film (commercially availablefrom E. I. du Pont de Nemours and Company of Wilmington, Del., U.S.A),and Upilex™ film (commercially available from Ube Film Ltd., located inJapan)), PEN is a thermoplastic, as contrasted with PI, which is athermoset material. Thus, upon localized heating with pressure, PEN andother similar thermoplastic polyesters can be joined or welded withitself to make strong splices in reel format. Splicing is useful aftercutting and separating out defective circuits in a reel so as to jointogether only good parts in a reel or joining short reel sections ofparts (e.g., panels cut into reel strips) together in larger reels. BothPI and PEN circuits can be spliced together with separate adhesive andtape, but PEN circuits can be spliced together more simply without useof extra material, cost and handling by heating above the melttemperature of PEN and making a PEN-to-PEN joint. Compared to standardsplice methods, the melted joint is relatively invisible (magnifiedinspection would be needed to see if splice is present) and stronger perunit area, making for less circuit area waste, an advantage to suppliersand customers (handlers) of the circuits. The advantage of this heatedsplice is not limited in scope to printer flex circuit applications, andcan be applied to all circuit configurations manufactured from PEN orsimilar thermoplastic polymer or polyester material.

Current methods used to splice TAB (Tab Automated Bonding) andreel-to-reel circuits typically involve the formation of an adhesivepatch that consists of an overlay of adhesive backed by a non-adhesivepolymer film support material. For sufficient bond strength, significantextra tape patch areas on parts are frequently required to provide asufficiently strong adhesive bond to survive handling processes inmanufacturing that expose the reel to various stresses (e.g., highimpact forces of short duration to continuous and cycling stresses,temperature stresses). This type of patch can thus extend beyonddefective parts on the reel meant to be removed and render otherwisegood adjacent parts to be “defective” parts as part of the splicingprocedure.

In contrast, a “near-invisible” splice (FIG. 3) requires less area andno extra material and offers the possibility of eliminating thedestruction of good parts to make a splice. As compared to typical tapesplices of 0.375 to 0.750 inch widths, splices of similar and sufficientstrength can be made in an area approximately 0.040 inch wide. More orless area could be used depending on the strength requirements of asplice.

FIG. 3 shows a continuous TAB style strip of circuits 100 with oneindividual circuit 200 being positioned to be spliced to the strip inaccordance with the present invention. A successfully welded splicejoint is shown at 300, wherein the splice joint is fully containedbetween adjacent circuits 100 without adverse effect.

Splices are commonly made in sections of tape containing circuits wheredefects occur. In prior art splicing techniques utilizing tape patches,a defective section of circuits is cut out and removed while leaving aportion of a defective part at each end to be rejoined with the tapepatch. This method leaves a defective part to be used as the joiningmember of the strip. In some cases, where good parts are dispersedwithin bad parts, manufacturers often cut out the good with the bad toreduce the quantity of splices. The “near-invisible” splice technique ofthe present invention does not result in any lost parts and thereforeresults in higher yield since no good parts are required to be removedor destroyed.

When producing reel-to-reel and TAB products with prior art technology,manufacturers typically allow a set number of defects to remain in thereel to reduce the quantity of splices. With “near-invisible” splicing,reels can be produced with 100% good parts without regard to the numberof splices, thereby the customer is provided with an exact number ofgood parts per length of material.

Producing reel-to-reel and TAB products requires expensive specializedequipment to process long rolls of material through the many stepsrequired for circuit manufacturing. Manufacturers that produce circuitsin panel form may find the cost of equipment prohibitive and may beunable to supply customers that require product to be delivered incontinuous reel form. A further advantage of the “near-invisible” spliceis that the length of the base raw material has no bearing on the lengthof the final TAB or reel-to-reel product being produced. Circuits can beproduced in panel form comprising either individual parts or individualshort strips of parts that can be joined together to form a continuouslength of product. The joining operation can be done on a single pieceof equipment at the final stage of the production process therebyreducing the cost of equipment to the manufacturer to enter thereel-to-reel and TAB market. These offer manufacturing cost andthroughput and others technological advantages over manufacturers solelyusing reel-to-reel equipment. Moreover, use of an adhesive strip asdetailed in FIG. 4 extends the benefit of this splicing method to othertypes of flex circuits, including those fabricated with PI and PEN.

Based on experiences with pressure sensitive tape adhesion, we suspectthat splices requiring additional strength beyond that obtained with aPEN to PEN, or similar thermoplastic polymer or polyester, heated jointmay be made stronger with the addition of a third component insertedbetween the PEN films at the joint. This third layer could be a narrowfilm strip (shown as 400 in FIG. 4) of pressure sensitive adhesive, ahot melt or heat re-flow adhesive material or one of several otherbonding techniques suitable for joining films together. The design ofthis type of modified splice joint is expected to enjoy all thepreviously detailed benefits of sufficient strength in a small area thatthe narrow welded splice joint width of the “near-invisible” spliceallows. And, can be extended to non-thermoplastic flex circuits material(e.g., polyimide).

EXAMPLES

1. Diffusion of generic inks through polyimide (PI), polyethyleneterephthalate (PET), and polyethylene naphthalate (PEN) base materialfilms.

Inks that are typically used in ink jet printers are polar liquidscomprised of a mixture of solvents, pigments, dyes, and/or water.Preferred flex circuit substrates, in accordance with the presentinvention, for ink jet printing are expected to have lower inkpermeability to avoid ink egress through the film into the vicinity ofthe metal conductors. Ink chemical make-ups vary widely and, as aresult, each will have its own permeability through the differentsubstrate materials. Ink permeability can be measured empirically or canbe estimated from known water and oxygen permeability and absorptionvalues as set out below in Table 3. PI, PET, and PEN are substrate orbase film materials. Table 3 compares some important physical propertiesof the three materials as they relate to permeability of the substrates.TABLE 3 Substrate Properties of PI, PET and PEN Related to InkPermeability and Absorption. Range of values for PI (in parentheses:Value for specific values for certain PEN Kapton products, (specificallyavailable from E.U. du Value Teonex Q83, Physical Pont de Nemours andfor avail. From Property Company) PET DuPont-Teijin) H2O    4-28 (22 forKapton 21-26 9.5 Permeability HN, 5 for Kapton E) (g/m²/day) H₂OAbsorption 1.8-3.7 (3 for Kapton 0.4-0.8 0.3 (weight %) HN, 1.8 forKapton E) O₂ Index (%)   4-114 (38 for Kapton 18 22 HN)

Absorption of polar substances like water in substrates can provide amodel to aid understanding about the suspected affinity and ease ofincorporation of other polar substances (like ink) inside substrates.This may also predict the ease of transport of polar substances throughsubstrates. Oxygen permeability is a common gage that can be used torank the general diffusivity of small gas molecules through differentsubstrates. This can also be a valuable aid when theoreticallyestimating permeability of larger molecules, such as ink constituents,through different substrates.

As can be seen in Table 3, the oxygen permeability values for PEN andPET, another polyester, are generally less than those for PI.

It is understood that pH differences generally increase reactivity asthe pH moves away from neutral (7 pH). Some PIs are known to be prone tochemical etching in strong base (e.g., Kapton E is known for having highetch rates in KOH and NaOH). That is, such etchable PI will have highpermeability and reactivity for any inks with pH approaching 14.Polyesters, on the other hand, are not known to be chemically etchedwith strong base solutions. As such, such polyesters as PEN and PETshould have less variability to expected values of permeability than PIacross inks of different pH values.

The permeability of two representative inks (in a range of high andneutral/low pH) were measured through various PEN, PI and PET substratebase films, which are suitable for fabrication into dimensionally stableflex circuitry, by monitoring ink weight-loss curves at 60 deg C.Exposure to inks with different pHs for long periods of time (e.g.,weeks) at temperatures of circa 60 deg C. is a typical test procedureused to accelerate failures in a print head environment (e.g.,previously referenced Japanese patent HEI 10[1998]-158582). For thesepermeability tests, ink was placed into metal cups with 6 cm diametercircular openings that were sealed with PI, PET, or PEN 50 micron thick“membrane” films from different manufacturers. Weight loss was monitoredversus time at intervals between 24 to 150 hours up to 870 or 1000hours. Linear regression, through which high correlation coefficients(in all cases greater than 99.5%) were obtained, was used to determinebest fit slopes for each film experiment, then slopes were averaged forthe different film types.

Both PEN films and one PET film tested had consistently lower inkpermeability than most PIs (see Table 4 below), thus indicating superioror at least approximately equivalent resistance to ink transport thanthe PI films. Thus, PEN and other polyesters with low permeabilityprovide advantageous properties for use with ink jet printer cartridges,as ink is not able to diffuse as quickly through PEN, PET or similarpolyesters as through most PIs. TABLE 4 Permeability of RepresentativeInks of Different pHs through PEN, PI and PET films. Permeability (g/m2-Different materials Permeability (g/m2- day) of Neutral/Lowmanufacturers' day) of High pH pH (pH < or equal Base thermallystabilized (pH >8) Ink to 7) Ink through film flexible films through thefilms films PEN 1 4.40 8.32 2 4.43 6.23 PI 1 6.39 7.60 2 26.7 25.3 323.1 24.1 PET 1 9.6 10.1

PEN 1, in Table 4 above, comprises a Teonex® Q83™ film, commerciallyavailable from DuPont-Teijin Films U.S Limited Partnership, located inHopewell, Va., U.S.A. PEN 2 comprises a Skynex® NX10L film, commerciallyavailable from SKC Co., Ltd in Seoul, Korea. PI 1 comprises a Kapton®200E™ film, commercially available from E. I. du Pont de Nemours andCompany, located in Wilmington, Del., U.S.A. PI 2 comprises a Kapton®200HN™ film, also commercially available from E. I. du Pont De Nemoursand Company, located in Wilmington, Del., U.S.A. PI 3 comprises aApical® 200NP™ film, commercially available from Kaneka High TechMaterials, Inc. of Japan. And, PET 1 comprises a Skyrol® AH82L film,commercially available from SKC Co., Ltd in Seoul, Korea. The high pHink listed in the table is Encad K208163-4GS ink having a pH of about8.56, commercially available from Big Systems, Inc. of Butler, Wis.,U.S.A. The low pH ink is Encad Y208163-3 GS ink having a pH of about6.8, also commercially available from Big Systems, Inc. of Butler, Wis.

The measurements shown in Table 4 are consistent with expected inkpermeability rankings between the three film types based upon theproperties set out in Table 3. PEN, in particular, is noted as being ahalf to full order of magnitude less permeable than most polyimides assuggested by the moisture and gas permeability values. The PET filmtested that had even lower permeability than expected (about 10 for inkversus 21-26 for water permeability), which means that PET is alsoeffective for ink jet printer flex circuit base substrate film. Moistureabsorption effects on the adhesion of metal, adhesives and otherlaminate constituents and cover coat adhesion properties to PEN and PIare also discussed in Examples 2 and 3 below.

2. Adhesion retention of metal to ipolvimide (PI) versus polyethylenenaphthalate (PEN) base materials upon exposure to representative inks.

As supported in Example 1 above, PEN and other polyesters have beenfound to be less permeable to representative inks than many PIs.Excessive permeability can be detrimental to metal adhesion in flexiblecircuit applications. Once the ink has diffused into themetal-to-polymer interface, according to its concentration levels, itcan attack the metal directly or weaken and break metal-to-polymer bondsor cause dendritic growth (as discussed above in the Background sectionabout ink properties and shorting and other failure mechanisms). It is,therefore, advantageous to use a substrate material with a lowerpermeability, as it will yield greater adhesion retention over time.However, it is also critical to assure the ink resistance of themetal-to-substrate bond when ink, even in low concentration, comes intocontact with it after diffusing through the substrate. The purpose ofExample 2 is, therefore, to quantify the resistance of themetal-to-substrate bond to different inks and to different ink exposuretimes on different types of PEN and PI flex test circuits with 1 ouncecopper thickness, which is used in most printer flex designs. The effectof moisture absorption properties (mentioned in Example 1) on metaladhesion to PEN and PI circuits is also discussed in this Example.

Bare circuits for two types of circuit designs were fabricated in orderto measure adhesion retention using two different methods. In the firstmethod, ¼-inch (6.35 mm) wide copper traces were patterned from PEN andPI laminates, and a 90 degree peel strength of the trace from thesubstrate was directly measured as a function of ink exposure time. Inthe second method, much finer width (75-100 micron) copper traces werepatterned from the same PEN and PI laminates, and circuits wereinspected for delamination as a function of ink exposure time. The firstmethod had the advantage of quantifying the adhesion (peel strength)retention of the metal from the base substrate film material directly,but the second method had the advantage of the test circuits having amore representative product design, as the final product typically hasminimum trace width dimensions ranging from 50 to 100 microns. With bothcircuit designs, traces were formed by aqueous etching in CuCl₂ (cupricchloride) using photoresist masking method (application, expose, aqueousdevelop of resist mask, stripping after etching) from initialcopper-on-polymer “laminates.” The term “laminates” includes bothadhesive-based and adhesive-less materials, that is copper depositioneither through lamination or sputtering/plating operations. Samples madefrom different circuit and material types were peel tested for initialadhesion. Additional samples were placed into glass jars filled withrepresentative inks (the same inks as used in Example 1, havingdifferent pH values) in an oven at 60 C., the same standard temperatureused to accelerate failures relating to ink exposures in Example 1.Samples were removed from the jars at approximately weekly intervals forup to 1000 hours and tested for adhesion retention over time.

With the ¼-inch trace circuits, peel force versus time were measuredwith a 100 pound load cell mounted on a “Chatillon” peel test fixturewith the substrate attached PEN or PI side face down to a German wheel,while the metal traces (separated for a short length before the peelforce was monitored) were held in grips anchored to a crossbar, and wereindividually pulled at 2 in/min crosshead speed. Unless otherwiseindicated, the failure mode was either an adhesive failure of thecopper-adhesive or adhesive-base film interfaces or a cohesive failurein either the adhesive or base film layer for the adhesive basedcircuits or either an adhesive failure of the copper-tie layer or tielayer-base film interfaces or a cohesive failure in either the tie layeror base film layer for the adhesive-less circuits. A failure modebetween the base material layer and the German wheel (e.g., in thebonding adhesive) was sometimes observed, which indicated a much lowerpeel test value than any of the common failures described above, whichfailure mode was unacceptable as not representing the truematerials-based peel strength and thus was not included in thisanalysis.

For the fine-lined circuits, three samples per circuit material set wereused and they were placed back in ink for additional ink exposures andre-inspections, since it was a non-destructive test. Fine-lined circuitswere considered to have failed when traces (or cover coat forcover-coated circuits in Example 3) became delaminated completely fromthe polymer substrate during the inspections. In all cases, samples wereremoved from the ink and washed with deionized water at circa 20 deg C.prior to inspection or peel testing. For peel testing, a destructivetest, three samples were used for each week of ink exposure conditionand failure modes were verified on the peeled samples after the test.

FIG. 5 is a bar graph showing the results of percent peel strengthretained in high pH (>8) ink at 60 C at weeks 0-8 for samples of PEN andPI applied to ¼-inch wide circuit traces. FIG. 6 is a bar graph showingthe results of percent peel strength retained in neutral/low pH (<7) inkat 60 C. at weeks 0-8 for samples of PEN and PI applied to ¼-inch widecircuit traces. In FIG. 5, adhesion retention was defined relative tothe 0 week peel strength value, except for the PEN-1 laminate, in whichcase the values for weeks 0 through 3 were averaged to define the 100%point. In FIG. 6, there are some missing values. Samples were taken atthe missing weekly intervals, but no good measurements resulted. Asdescribed previously, data were prone to noise in the testing process,and system, which contributed especially to some readings above 100% inboth Figures.

The adhesion of the ¼-inch wide traces (see FIGS. 5 and 6) was lesssensitive to ink exposure than that of the narrower traces (see Tables 5and 6 below). With the circuits with ¼-inch traces, both adhesive-lessbased PI circuits and the adhesive-based PEN circuits retained about thesame percentage of peel strength (see FIGS. 5 and 6) with post-6 weekpeel retentions varying only between about 80 to 100% (roughly the sameconsidering the noise inherent to this destructive peel technique).

However, with the circuits with fine traces, the failure times of the PIcircuits were always shorter than the failure times of the PEN circuits,especially with exposure to high pH ink (1 and 2 weeks versus greaterthan 8 weeks). In summary, the results show that many varieties ofPEN-based circuits survived long ink exposure conditions, which includeddifferent PEN film raw material sources and/or manufacturers, differentPEN laminate manufacturers, different adhesive formulations includingbut not limited to modified epoxies and polyester-epoxy blends, and theinclusion or non-inclusion of dyes and flame retardant additives to theadhesive part of the laminate. Moreover, a comparison of FIGS. 5 and 6also show that pH has very little effect on the circuit traces where thebase material comprises a PEN material regardless of the other factors,which is a significant advantage for use in print head flex circuits.

Tables 5 and 6 show the results of testing 3 samples of each type offilm. The percentage of the failures, out of 100%, reflects thepercentage of the 3 samples that 10 failed. A weekly failure time isprovided when one of the three failure percentages (33, 67 and 100%)occurred. As seen in Tables 5 and 6, PEN consistently performed well inthe application with inks of both low and high pH showing littlevariability across the pH range tested. There was not as much of adifference in PEN's performance from PI's at low pH ink exposure,although the PEN variations performed consistently as well if not betterthan PI. At exposure to higher pH inks, PEN performed much better thanPI consistently over the PEN variations. TABLE 5 Bare Circuit Survivalin High pH (pH >8) Ink at 60 deg C. PI (PI) or Copper 33% 67% 100% PENFilm Laminate Adhesive Dye in FR in Failure Failure Failure film Manuf'rManuf'r Chemistry Adhesive Adhesive Time Time Time PI-1 1 1 na na Na 1week 2 weeks 2 weeks PEN-1 1 1 Polyester yes Yes >8 weeks epoxy-1 PEN-52 2 Modified no Yes >8 weeks epoxy-1 PEN-2 1 1 Polyester yes No >8 weeksepoxy-2 PEN-3 1 1 Polyester no Yes >8 weeks epoxy-3 PEN-4 1 1 Polyesterno No >8 weeks epoxy-2 PEN-6 1 3 Modified no Yes >7 weeks epoxy-1

TABLE 6 Bare Circuit Survival in Neutral/Low pH (pH less than or equalto 7) Ink at 60 deg C. PI (PI) or Copper 33% 67% 100% PEN Film LaminateAdhesive Dye in FR in Failure Failure Failure film Manuf'r Manuf'rChemistry Adhesive Adhesive Time Time Time PI-1 1 1 na na Na   2weeks >8 wks >8 wks PEN-1 1 1 Polyester yes Yes >8 weeks epoxy-1 PEN-5 22 Modified no Yes >8 weeks epoxy-1 PEN-2 1 1 Polyester yes No >8 weeksepoxy-2 PEN-3 1 1 Polyester no Yes >8 weeks epoxy-3 PEN-4 1 1 Polyesterno No >8 weeks epoxy-2 PEN-6 1 3 Modified no Yes >7 weeks epoxy-1

The source of the materials in FIGS. 5 and 6 and Tables 5 and 6 areprovided below. PI-1 was an adhesive-less PI, and is specificallyKapton-E film, commercially available from E. I. du Pont de Nemours andCompany of Wilmington, Del., U.S.A. The film included copper sputteredover a chromium sputtered tie layer and then electroplated up to 35micron thickness according to U.S. Pat. No. 4,863,808. PEN-1 was apolyester-epoxy adhesive with both a dye and a flame retardant, on whichMultek Flexible Circuits Inc., Sheldahl Technical Materials Division(Northfield, Minn.) laminated 1 oz Cu foil with 0.7 mil thick A477adhesive (called “Polyester-epoxy-1”) to the 1 mil thick PEN film. PEN-2includes a polyester-epoxy adhesive with a dye, but without a flameretardant, on which Multek Flexible Circuits Inc., Sheldahl TechnicalMaterials Division (Northfield, Minn.) laminated 1 oz Cu foil with 0.7mil thick A523 adhesive to the 1 mil thick PEN film. The polyester-epoxyblend adhesive is a different formulation than above (thus identified inthe Table as “Polyester-epoxy-2”). PEN-3 included a polyester-epoxyadhesive without a dye but with a flame retardant, on which MultekFlexible Circuits Inc., Sheldahl Technical Materials Division(Northfield, Minn.) laminated 1 oz Cu foil with 0.7 mil thick A478adhesive to the 1 mil thick PEN film (called G1910). The polyester-epoxyblend adhesive is a different formulation than either incorporated intothe other Sheldahl laminates above (thus identified in the Table as“Polyester-epoxy-3”). PEN-4 included a polyester-epoxy adhesive withouta dye and without a flame retardant. Multek Flexible Circuits Inc.,Sheldahl Technical Materials Division (Northfield, Minn.) laminated 1 ozCu foil with 0.7mil thick A523 adhesive to the 1 mil thick PEN film. Thepolyester-epoxy blend adhesive is a different formulation than some ofthe other adhesives (thus identified in the Table as“Polyester-epoxy-2”). PEN-5 included a modified epoxy adhesive withdifferent chemistry than the Sheldahl laminates listed above. There isno dye, but a flame retardant was added to the laminate. This is alaminate like GTS 5670 (available from GTS Flexible Materials Ltd.,Berkshire, United Kingdom) laminated 1 oz Cu foil with 1 mil thickadhesive. The differing appearance and properties of the adhesive and Cufoils indicate different raw material manufacturing sources from thelaminates made by Multek Flexible Circuits Inc., Sheldahl TechnicalMaterials Division and GTS Flexible Materials Ltd., although the PENfilm from these two laminate manufacturers may be the same. PEN-6 fromTaiflex Scientific Company, Ltd., Kaohsiang, Taiwan uses a Teonex PENfilm (DuPont-Teijin Films™ Teonex® Q83™, available from DuPont TeijinFilms US Limited Partnership, in Hopewell, Va., U.S.A.) that has thesame appearance as PEN base films made by Multek Flexible Circuits Inc.,Sheldahl Technical Materials Division, but with a different copper foil(available from Furukawa Circuit Foil Company, Ltd., located inImaichi-City, Tochigi-prefecture, Japan) and a different adhesivechemistry (a modified epoxy that has different appearance and propertiesthan the modified epoxy included in the laminate made by GTS FlexibleMaterials Ltd.). The superior longevity of the bare PEN circuits (barecircuits have metal traces exposed to the environment and are not coatedwith a cover layer or cover coat) was at least partly attributed togreater potential adhesion retention as a result of physical andchemical bonding differences between the adhesive-based materials thanadhesive-less circuits. With the adhesive-based systems adhesionretention will depend on ability of the adhesive and its interfaces towithstand attack from the ink components. Any ink component includingmoisture that permeates through the base substrate material canpotentially attack the adhesive. Just as important, adhesion may beweakened at either or both adhesive-substrate or adhesive-metalinterfaces as a result of absorbed moisture or any part of the inksolution that swells the adhesive or the base material thereby creatingstress in the metal-substrate stack-up. Interfacial bonds or bonds inthe internal bulk structure of the adhesive are susceptible to attack bythe ink components. For this reason, it is expected that differentadhesive systems used in conjunction with different base materials willhave better reliability over others. Copper foil adhesion to theadhesive will also be impacted by copper roughness and surface interfacetreatments such as Zn—Cr. It is expected that a greater copper roughnessor “tooth” will be more reliable than smoother copper because of greatersurface area for bonding and Zn—Cr coatings will improve adhesionretention due to galvanic protection of the copper. All of theadhesive-based PEN circuits tested had copper foils in the raw laminateswith adhesion, barrier and stabilization treatments, so these arepreferred in the invention. However, other current or future developedadhesion, barrier and stabilization treatments are also contemplated.Because of long survival in the tests, adhesives with modified epoxy andpolyester epoxy blend chemistries are also preferred, although othersare expected to work sufficiently

Unlike the adhesive-based systems, the adhesive-less materials have aCu-tie coat-substrate stack-up. The adhesive-less materials are notexpected to have as good of adhesion retention since the copper-tie coat(chromium, monel, etc.) couple may be susceptible to galvanic corrosion(e.g., chromium is more noble than copper), but are expected to besufficient for some ink jet applications.

3. Cover coated circuits comparison of those made from polyimide (PI)versus polyethylene naphthalate (PEN) base materials.

Some fine-lined bare circuits with metal traces representative ofprinter flex product designs described in Example 2 were cover coatedwith two representative cover coats and exposed to the same acceleratedink environments of 60 deg C. for up to 1000 hours as in Example 1 and2. The different material types, procedure and failure criteria(includes delamination of cover coat) have been described in Example 2.The cover coat material and process conditions and ink performancesummary are listed in Tables 7-9. Tables 7-9 also show the failure timein weeks at which failure in one of three percentages of failure for thethree samples tested (33, 67 and 100%). As can be seen, PEN-film basecircuits comparatively survived at least as long as PI film basedcircuits, in most cases longer. With high pH ink, circuits made from all5 PEN materials that were cover coated with epoxy-acrylic resin material#1 survived longer than the PI based circuits cover coated similarly(see Table 7) and circuits made from 4 out of 5 PEN materials that werecover coated with epoxy-acrylic-resin based material #2 survived longerthan the similarly coated PI based circuits (see Table 8). Withneutral/low pH ink, all PEN and PI circuits survived the 7-week durationof the test without failures (see Table 9). TABLE 7 Cover-coated CircuitSurvival in High pH (pH >8) Ink at 60 deg C. - Cover Coat 1. Copper 33%67% 100% PI (PI) or Film Laminate Adhesive Dye in FR in Failure FailureFailure PEN film Manuf'r Manuf'r Chemistry Adhesive Adhesive Time TimeTime PI-1 1 1 na na Na   7 weeks 7 weeks >7 weeks PEN-1 1 1 Polyesteryes Yes >7 weeks epoxy-1 PEN-5 2 2 Modified no Yes >7 weeks epoxy-1PEN-2 1 1 Polyester yes No >7 weeks epoxy-2 PEN-3 1 1 Polyester noYes >7 weeks epoxy-3 PEN-4 1 1 Polyester no No >7 weeks epoxy-2

TABLE 8 Cover-coated Circuit Survival in High pH (pH >8) Ink at 60 degC. - Cover Coat 2. Copper 33% 67% 100% PI (PI) or Film Laminate AdhesiveDye in FR in Failure Failure Failure PEN film Manuf'r Manuf'r ChemistryAdhesive Adhesive Time Time Time PI-1 1 1 na na Na 5 weeks 6 weeks 6weeks PEN-1 1 1 Polyester yes Yes 6 weeks 6 weeks 6 weeks epoxy-1 PEN-52 2 Modified no Yes 6 weeks 6 weeks 6 weeks epoxy-1 PEN-2 1 1 Polyesteryes No 5 weeks 5 weeks 6 weeks epoxy-2 PEN-3 1 1 Polyester no Yes 6weeks 6 weeks 6 weeks epoxy-3 PEN-4 1 1 Polyester no No 6 weeks 6 weeks6 weeks epoxy-2

TABLE 9 Cover-coated Circuit Survival in Neutral/Low pH (pH <7) Ink at60 deg C. - either Cover Coat 1 or Cover Coat 2. Copper 33% 67% 100% PI(PI) or Film Laminate Adhesive Dye in FR in Failure Failure Failure PENfilm Manuf'r Manuf'r Chemistry Adhesive Adhesive Time Time Time PI-1 1 1na na na >7 weeks PEN-1 1 1 Polyester yes yes >7 weeks epoxy-1 PEN-5 2 2Modified no yes >7 weeks epoxy-1 PEN-2 1 1 Polyester yes no >7 weeksepoxy-2 PEN-3 1 1 Polyester no yes >7 weeks epoxy-3 PEN-4 1 1 Polyesterno no >7 weeks epoxy-2

Cover Coat 1 is TF200FR2 cover coat based on epoxy-acrylic resinchemistry, screen printed and cured according to supplier's standardpublished recommendations (Taiyo America, Inc., Carson City, Nev.,U.S.A.). Cover Coat 2 is NPR-5 epoxy-acrylic resin based cover coat,screen printed and cured according to supplier's standard publishedrecommendations (Nippon Polytech Corp., Tokyo, Japan).

The results in Tables 7-9 above show that PEN is a good choice ofmaterial for the application. It is good regardless of the type of PENused. PEN is at least as good as PI, and in many cases it performsbetter than PI in this application.

4. Capability of PEN circuits to be fabricated with all the required,characteristic ink jet printer flex circuit design features.

Besides the fine lined, cover coated test circuits used to evaluate inkperformance in Examples 2, 3, and 5, 1 ML and 2 ML PEN circuits, havingall features generally characteristic of printer cartridge circuitdesigns, were fabricated successfully using all of the various PEN basefilm laminate materials (PEN-1 through PEN-6). The 1 ML fabricationmethods included subtractive etching of copper using a negativephotoresist that was accomplished in the same way as was described inExample 2. Patterned removal of PEN and adhesive in order to accessbackside metal features for terminal connections and to produce spanningconductor features was accomplished by laser ablation. Metal surfacecleaning of laser residues was accomplished by a combination of oxygenor oxygen/CF4 plasma, chemical and/or electrolytic (cathodic or anodic)cleaning. All copper is preferred to be plated with electrolytic gold.Cover coats were preferably applied by a screen printing process, butphotoimageable cover coats were also applied successfully.

When backside access to front-side metal circuit features is required,following laser ablation (both UV and CO2 types used successfully), thefollowing methods can be used in combination to clean the metalsufficiently for post-plating (e.g., electrolytic gold): plasma (both O2and O2/CF4 methods used successfully), chemical cleaning withsulfuric-acid-based microetch baths (e.g., persulfate, peroxy-sulfuricsuccessfully used), and electrolytic cleaning (e.g., both sodium andpotassium hydroxide based cleaners were successfully used).

Two-metal (2 ML) adhesive-based PEN circuits were also made by laserdrilling 30-50 micron diameter vias for frontside to backside circuitlayer access, in combination with all the 1 ML process steps describedabove for patterning metal layers and dielectric features. PEN itselfwithout adhesive was also ablated or etched in order to be able tocreate vias, backside access pad and spanning conductor features for usein adhesive-less PEN circuit constructions. Chemical etch rates weredetermined of 50 and 75 micron thick PEN films (Teonex® Q83 film,available from DuPont-Teijin Films US Limited Partnership of Hopewell,Va., U.S.A.) to be as high as 12 micron/minute by measuring weight lossafter heated exposure to various monoethanol amine solutionconcentrations in different basic solutions (NaOH and KOH based). Usinga metal mask type of process as described previously has been found tocreate acceptable PEN sidewall profiles, because both KOH and NaOH haveproven effective in chemically etching (isotropic etch at similar etchrates) PI patterns with acceptable sidewall profile in PI flex circuitswith this type of masking approach, although the unzipping mechanism(i.e., mechanism of breaking the chemical bonds in a polymer) isdifferent on PI than with PEN.

Direct PEN metallized (chromium tie layer sputtered andelectrolytic-plated) adhesive-less PEN material with circa 1 lbs/inadhesion values is available commercially from Multek Flexible Circuits,Inc., Sheldahl Technical Materials Division, located in Northfield,Minn., U.S.A., as an option to adhesive-based laminates provided by them(e.g., see PEN-1 through PEN-4 laminate descriptions). Also, direct PENmetallization by lamination of PEN to Olin Corporation, Brass Division's(located in East Alton, Ill., U.,S.A.) commercially available copperfoils as created in a small lamination press gave similar levels ofadhesion.

5. THB and biased ink immersion electromigration testing.

To test for the possibility for catastrophic electromigration failuresoccurring over extended product lifetimes, circuit samples withproduct-representative minimum, 75 micrometer (nominal) trace width andspaces, with the traces oriented parallel to each other, were fabricatedon different base flexible films, both adhesive-less and adhesive-basedwith different adhesive types (similar to those described in Example 2)and were tested in an accelerated test environment. These circuits werecover coated with the same two materials and with the correspondingprocesses described in Example 3. The patterned copper trace can bedescribed as a “comb,” such that every other trace is electricallyconnected together along one side to a large (5 mm square) terminal padand the adjacent, every-other set of traces are electrically connectedto each other along the other side and connected to another largeterminal pad. The two pads were soldered to wires connected to anexternal DC power supply, thereby enabling adjacent traces to bealternately biased, while the circuits were exposed to two separate testenvironments: a standard accelerated temperature-humidity-bias (THB)stress environment and a bias, ink environment. As a reference, aJapanese patent (referenced earlier, Patent Journal (A) Kokai PatentApplication no. HEI 10[1998]-158582)) describes a similar THB test for500-1000 hours, which results were correlated to a survival time inbiased ink immersion testing (similar to the THB test, but instead of an85C./85% RH, immersion in liquid ink was used: 5 to 19.25 volt range,144 to 672 hours test duration range, where 2 samples per circuitcoating material were evaluated), and which was used to test differentcover coat materials on adhesive-less PI circuits.

Likewise in this study, THB JEDEC (conditions of/85% RH/10volts/1000hours, see EIA/JESD Standard Test Method A-101-B, “Steady StateTemperature Humidity Bias Life Test,” Apr. 1997; 5 circuits per laminatematerial type were evaluated here) and biased ink immersion (conditionsof 60 deg C./320 hours/5 volts; 2 circuits per laminate material typewere evaluated here) were also used to approximate the environmentalexposure in liquid ink of the product accelerated over its lifetime.Gathering results with different circuit material types enabled arelative comparison between materials about electrical performance in anenvironment closer to the application (circuits exposed to liquid inkrather than to just humidity), which complemented the relativecomparison between materials that the THB test provided.

The THB dryout conditioning (after the bias/enviromnental exposure timewas completed and just prior to making the post-stress resistancereadings) was 48 hours in air (see A-101-B test method description), butthe biased ink immersion dry-out condition was for 3 hours at 105 degC., in order to allow for as much of the absorbed liquid ink ionic andnon-ionic volatiles (water and other more volatile ink components)opportunity to escape.

Performing the bias test in ink had as a basis the standard test methodfor monitoring CAF growth (sometimes called dendrite formation) withtemperature-humidity (e.g., 85 deg C./ 85% relative humidity) and bias(1.8 volts and higher) stresses that simulate what real circuits incomputers and other electronics applications experience only acceleratedin time. In the THB test, however, the dry-out period (48 hours) permitsthe circuits to return reversibly to their initial, steady-stateenvironmental conditions, as long as dendrites have not formed to bridgethe adjacent traces at any point in the comb design. Inherent to basesubstrate and cover coat materials, the insulation resistance decreaseswith humidity for the accelerated THB test environment, so the dry-outperiod allows for the circuits to return to the same steady-stateenvironmental conditions for the post-stress resistance readings andoffers the best comparison between initial and final resistance values.

In contrast and inherent to the biased ink immersion test, all ionicconstituents in the ink that have been absorbed into the region of theadjacent conductors, but are for example less volatile, are not removedeffectively by any dry-out procedure and they contribute to lowering thepost dry-out insulation resistance. Therefore, returning of the stressedsamples to the same condition as they were initially was impossible, andexact comparisons between post- dry out and pre-stress insulationresistance values could not strictly be made, although the dry-outconditioning procedure used sought to return the circuits to as dry of asteady state environment as they experienced before being immersed inliquid ink before the test. With both the THB and biased ink immersiontests, an ohmimeter accurate and sensitive up to about 10E13 ohm wasused to measure both initial and post-dry out insulation resistancevalues of the circuits at 50 volts of constant bias for 60 seconds.

The failure criteria for both the biased ink immersion and the THB testwere set as follows. If individual comb circuits exhibited a much lowerpost-stress than initial resistance (e.g., low ohms might indicate ashort or metal bridge), these circuits have failed either test, but ifcircuits exhibited (1) high post-stress resistance values, above 10E04or 10E6 ohms for the biased ink immersion test, or (2) similarresistance, higher or within 1 order of magnitude lower than thepre-stress reading, for the THB test, the circuits successfully passedthe test.

For the THB test, all circuits made from all material types passed,because final dry-out resistance was never less than an order ofmagnitude of the already high (above 10E9 ohms) initial resistance (seeTable 10 and 11) and most often was higher than the initial resistance.Thus, all PEN circuits, fabricated by multiple manufacturers(potentially as many as 2 PEN base film sources and definitely 3laminate manufacturers) with different adhesive chemistries (3 differentpolyester-epoxy blends and 2 different modified epoxies) and with orwithout inclusion of dyes or flame retardants performed equally as wellas the adhesive-less PI circuits, regardless of which representativecover coat (#1 or #2) was used. TABLE 10 THB Results for PI and PENBased Circuits with Cover Coat 1. PI (PI) or Copper Pass PEN FilmLaminate Adhesive Dye in FR in or film Manuf'r Manuf'r ChemistryAdhesive Adhesive Fail PI-1 1 1 na Na Na Pass PEN-1 1 1 Polyester YesYes Pass epoxy-1 PEN-5 2 2 Modified No Yes Pass epoxy-1 PEN-2 1 1Polyester Yes No Pass epoxy-2 PEN-3 1 1 Polyester No Yes Pass epoxy-3PEN-4 1 1 Polyester No No Pass epoxy-2 PEN-6 1 3 Modified No Yes Passepoxy-1

TABLE 11 THB Results for PI and PEN Based Circuits with Cover Coat 2. PI(PI) or Copper Pass PEN Film Laminate Adhesive Dye in FR in or filmManuf'r Manuf'r Chemistry Adhesive Adhesive Fail PI-1 1 1 na na Na PassPEN-1 1 1 Polyester yes Yes Pass epoxy-1 PEN-5 2 2 Modified no Yes Passepoxy-1 PEN-2 1 1 Polyester yes No Pass epoxy-2 PEN-3 1 1 Polyester noYes Pass epoxy-3 PEN-4 1 1 Polyester no No Pass epoxy-2 PEN-6 1 3Modified no Yes Pass epoxy-1

For the biased ink immersion test, all circuits made from all materialtypes likewise passed (see Table 12 for circuits with Cover Coat 1 andTable 13 for circuits with Cover Coat 2), because the post-stressresistance criteria were met and no electrical shorts or metal wereobserved during post-stress optical microscope inspections. TABLE 12Biased Ink immersion Results for PI and PEN Based Circuits with CoverCoat. 1. PI (PI) or Copper Pass PEN Film Laminate Adhesive Dye in FR inor film Manuf'r Manuf'r Chemistry Adhesive Adhesive Fail PI-1 1 1 na nana Pass PEN-1 1 1 Polyester yes Yes Pass epoxy-1 PEN-5 2 2 Modified noYes Pass epoxy-1 PEN-2 1 1 Polyester yes No Pass epoxy-2

TABLE 13 Biased Ink immersion Results for PI and PEN Based Circuits withCover Coat. 2 PI (PI) or Copper Pass PEN Film Laminate Adhesive Dye inFR in or film Manuf'r Manuf'r Chemistry Adhesive Adhesive Fail PI-1 1 1na na Na Pass PEN-1 1 1 Polyester yes Yes Pass epoxy-1 PEN-5 2 2Modified no Yes Pass epoxy-1 PEN-2 1 1 Polyester Yes No Pass epoxy-2

The above data show that equivalent performance of many PEN laminatesystems with a variety of adhesive and copper foil types versusadhesive-less PI based circuits under the conditions of these two biastests without any evidence for dendritic shorts. Moreover, PEN's lowerink permeability and moisture absorption than PI's is believed to makePEN circuits superior to PI circuits, although both are acceptable astested.

6. “Near-Invisible” Splice Strength Testing.

The evaluation of “near invisible” splices of thermoplastic polymericmaterial flex circuits include two variations of splice. One variationconsisted of overlapping two circuits end-to-end along a 24 mm width by0.040 inches and applying heat and pressure to the overlap area to forma “welded” PEN joint. The second variation consisted of overlapping twocircuits end-to-end along a 24 mm width by 0.040 inches with a strip ofthermoplastic adhesive between the two circuits at the joint. Heat andpressure were then applied to the overlapped area to reflow the adhesiveand bond the two circuits together. In both cases, the joint was “nearlyinvisible” and did not extend into the functional area of the part.

24 mm circuits tensile testing of adhesive-less and adhesive splicejoints was measured with a 10 kg load cell mounted on a “Chatillon” peeltest fixture with the substrate attached PEN circuit held in gripsanchored to a crossbar. There were 28 adhesive-less samples tested and46 adhesive samples tested. Individual circuits were pulled at 2 in/mincross head speed until the joint failed. The recorded value representsthe maximum pounds of force achieved at the point of failure in poundsof force per inch. All samples failed at the joint. For multiple samplestested, the average maximum force (lb f/in) was 6.22 for theadhesive-less samples and 14.78 for the samples with adhesive added.Samples tested comprised PEN base layers as a suitable thermoplasticpolymer for splicing with the understanding that any thermoplasticpolymer could be spliced as described. Both variations of the “nearinvisible” splice performed to acceptable levels for circuitmanufacturing where forces typically do not exceed 2 pounds per inch.The “near invisible splice” as tested occupied a 0.040 inch wide stripof material. This narrow strip allowed the splice to be made in a wastearea between adjacent parts and allowed 100% part yield as compared toother splice methods, such as for example a butt joint using pressuresensitive tape which would typically cover a much larger area and extendinto the actual circuit area thereby making the part a scrap part evenif no other defect was present. The adhesive-less splice has theadvantage of not requiring any additional material to make the splice,such additional material may not be as compatible with the circuitmanufacturing process as the base PEN material. The thermoplasticadhesive splice provides a stronger joint and can be used inapplications requiring a higher force.

1. A flex circuit for use in an ink jet printer, the flex circuitcomprising a flexible substrate comprising a polyester material layersupporting a plurality of metal conductors adhered along at least aportion of a first side of the substrate, the polyester materialcomprising a material suitable for use in an ink environment with lowerink permeability and moisture absorption than polyimide material.
 2. Theflex circuit of claim 1, wherein the polyester material of the substratecomprises PEN.
 3. The flex circuit of claim 2, further comprising atleast one opening provided through the PEN layer for providing access toat least one conductor.
 4. The flex circuit of claim 3, furthercomprising a metal access pad adhered on the first side of the PENsubstrate layer with the plurality of metal conductors, the metal accesspad being accessible from a second side of the PEN substrate layerthrough a patterned opening through the PEN substrate layer, and whereinat least one metal conductor is also accessible from the second side ofthe PEN substrate layer by way of another opening through the PENsubstrate layer.
 5. The flex circuit of claim 3, further comprising atleast one metal conductor adhered along at least a portion of a secondside of the PEN substrate layer and that is electrically connectedthrough a metal via extending through the PEN substrate layer to atleast one of the metal conductors on the first side of the PEN substratelayer.
 6. The flex circuit of claim 2, 4 or 5, further comprising anadhesive layer between the PEN substrate layer and at least one of themetal conductors for adhering them together.
 7. The flex circuit ofclaim 2, 4, or 5, wherein at least one of the metal conductors isadhered to the PEN substrate layer without an adhesive layer in between.8. A method of making a flex circuit for use in an ink jet printer, themethod comprising the steps of providing a flexible substrate includinga polyester material layer and adhering a plurality of metal conductorsto one surface of the substrate, wherein the polyester material issuitable for use in an ink environment with lower ink permeability andmoisture absorption than polyimide material.
 9. The method of claim 8,wherein the polyester material of the substrate comprises PEN.
 10. Themethod of claim 8, further comprising the step of patterning at leastone opening through the PEN layer for providing access to at least oneconductor.
 11. The method of claim 10, further comprising the step ofadhering a metal access pad on the first side of the PEN substrate layeralong with the plurality of metal conductors, the metal access pad beingaccessible from a second side of the PEN substrate layer through a firstopening patterned through the PEN substrate layer, and patterning asecond opening through the PEN substrate layer so that at least onemetal conductor is also accessible from the second side of the PENsubstrate layer by way of the second opening.
 12. The method of claim10, further comprising the step of adhering at least one metal conductoralong at least a portion of a second side of the PEN substrate layer andelectrically connecting the metal conductor on the second side by way ofa metal via extending through the opening of the PEN substrate layer toat least one of the metal conductors on the first side of the PENsubstrate layer.
 13. The method of claim 10, further comprising thesteps of providing a laminate of the PEN substrate layer and an adhesivelayer, patterning the laminate to provide at least one access openingthrough the laminate, adhering a metal layer to the PEN substrate layerby way of the adhesive layer, and then patterning the metal layer tocreate the plurality of metal conductors.
 14. The method of claim 9, 11or 12, further comprising the step of providing an adhesive layerbetween the PEN substrate layer and at least one of the metal conductorsfor adhering them together.
 15. The method of claim 9, 11, or 12,wherein at least one of the metal conductors is adhered to the PENsubstrate layer without an adhesive layer in between.
 16. A print headfor use in an ink jet printer comprising a printer and an ink cartridgeand a flex circuit connected electrically to the IC, the flex circuitcomprising a flexible substrate comprising a polyester material layersupporting a plurality of metal conductors adhered along at least aportion of the substrate, the polyester material comprising a materialsuitable for use in an ink environment with lower ink permeability andmoisture absorption than polyimide material.
 17. The print head of claim16, wherein the polyester material of the substrate comprises PEN. 18.The print head of claim 17, further comprising at least one openingprovided through the PEN layer for providing access to at least oneconductor.
 19. A method of joining a plurality of flex circuits togetherin series comprising the steps of: providing a plurality of unconnectedflex circuits, each having a flexible substrate including athermoplastic polymer material layer, wherein the thermoplastic polymermaterial is suitable for use in an ink environment with lower inkpermeability and moisture absorption than polyimide material, and eachflex circuit further having a plurality of metal conductors adhered toone surface of the substrate; and splicing one flex circuit to a secondflex circuit by overlapping at least a portion of the first and secondflex circuits together and applying heat and pressure sufficient tothermally bond the first and second flex circuits together in series.20. The method of claim 19, wherein the thermoplastic polymer materialof the substrate comprises a polyester.
 21. The method of claim 19,wherein the thermoplastic polymer material of the substrate comprisesPEN.
 22. The method of claim 19, further comprising the step ofinserting a strip comprising an adhesive on the overlapped portionbetween the first flex circuit and the second flex circuit prior tothermally bonding the first and second flex circuits together.
 23. Themethod of claim 19, wherein the first flex circuit is combined with oneor more additional flex circuits having the thermoplastic polymermaterial substrate layer in common.
 24. The method of claim 23, whereinthe thermoplastic polymer material of the substrate comprises apolyester.
 25. The method of claim 23, wherein the thermoplastic polymermaterial of the substrate comprises PEN.
 26. A method of joining aplurality of flex circuits together in series comprising the steps of:providing a plurality of unconnected flex circuits, each having aflexible substrate including a polymer material layer, wherein each flexcircuit further includes a pattern of metal conductors adhered to onesurface of the substrate; overlapping an edge portion of the flexiblesubstrate outside of the pattern of metal conductors of one flex circuitwith an edge portion of the flexible substrate outside of the pattern ofmetal conductors of another flex circuit; positioning a strip comprisinga thermally active adhesive within an overlapped portion between thefirst flex circuit and the second flex circuit; and splicing one flexcircuit to a second flex circuit by applying heat and pressuresufficient to thermally bond the first and second flex circuits togetherin series.
 27. The method of claim 26, wherein the polymer materiallayer of the substrate comprises polyimide.
 28. The method of claim 26,wherein the polymer material layer comprises a thermoplastic polymer andthe splicing step further comprises thermally bonding the flexiblelayers together along with the thermally active adhesive.
 29. The methodof claim 28, wherein the polymer material layer comprises PEN.